For each plant species, there exists a wide discrepancy in plant growth due to environmental conditions. Under most conditions, the maximum growth potential of a plant is not realized. Plant breeding has demonstrated that a plant's resources can be redirected to individual organs to enhance growth.
Genetic engineering of plants, which entails the isolation and manipulation of genetic material, e.g., DNA or RNA, and the subsequent introduction of that material into a plant or plant cells, has changed plant breeding and agriculture considerably over recent years. Increased crop food values, higher yields, feed value, reduced production costs, pest resistance, stress tolerance, drought resistance, the production of pharmaceuticals, chemicals and biological molecules as well as other beneficial traits are all potentially achievable through genetic engineering techniques.
Plant growth responds to the increased availability of mineral nutrients in the soil, but shoot and root growth respond differently. Moreover, a direct relationship between mineral nutrient availability and change of growth rate is rarely observed over a larger concentration range. This suggest that plant growth is limited materially by nutrients required for cell growth as well as by signaling pathways that control the rate of organ growth for the overall benefit of the plant. Although the components of these regulatory pathways have not been identified, they define two distinct avenues to potentially improve plant growth. It has been shown that enhanced accumulation of cyclin protein under control of the cdc2 promoter suffices to enhance root and overall plant growth under non-limiting conditions on growth media.
Plants rarely grow under optimal conditions. Plant growth can be limited by water availability, mineral nutrients and a short growing season. Drought tolerance in genetic variants of a given species is well correlated with the penetration depth of its root system into the soil. Fertilizers are often not optimally utilized because of insufficiently penetrating root systems. Although the induction of flowering can now be controlled, thereby extending the potential growth range of some important crop species, this does not in itself lead to increased biomass.
The ability to manipulate gene expression provides a means of producing new characteristics in transformed plants. For example, the ability to increase the size of a plant's root system would permit increased nutrient assimilation from the soil. Moreover, the ability to increase leaf growth would increase the capacity of a plant to assimilate solar energy. Obviously, the ability to control the growth of an entire plant, or specific target organs thereof would be very desirable.